Abstract: Large squeezing deformation of soft rocks occurs frequently in tunnels constructed in complex mountainous areas under high ground stress. Conventional rigid support structures typically crack, experience steel arch distortion, or fail because of the large deformation of surrounding rock. In this study, a rigid-flexible composite lining structure with a toughened primary support-polymer buffer layer-secondary lining is proposed to control the cracking and damage of the secondary lining and improve long-term disaster prevention adaptability. The load-bearing characteristics of the proposed rigid-flexible composite lining under large squeezing deformation are clarified. Subsequently, large-scale model loading tests are conducted for conventional and rigid-flexible composite linings with varying thicknesses. These tests are based on an independently developed tunnel lining loading test device. Finally, the strain evolution and crack development are monitored using digital image correlation and acoustic emission techniques, and the force-deformation characteristics, damage failure modes, and load-bearing and deformation capacities of the specimens are compared and analyzed. The test results demonstrate that (1) compared with conventional composite lining, the load-bearing capacity, deformation capacity, and toughness of the proposed rigid-flexible composite lining increase by 188%, 21%, and 115%, respectively; (2) the polymer buffer layer in the built synergistic system of toughened primary support-buffer layer-secondary lining in the proposed rigid-flexible composite lining exhibits a notable impact on the yielding capacity of the lining; and (3) the damage evolution of the rigid-flexible composite lining presents a four-stage progressive characteristic of primary support cracking, crack propagation, buffer layer compression, and failure.

Key words: tunnel engineering, large squeezing deformation, rigid-flexible composite lining, load-bearing characteristics, model test